Work in progress commit on saving OpenEXR with all render-layers and

passes in single file. Code is currently disabled, commit is mainly to
have a nicer method of excluding OpenEXR dependency from render module.
This should compile with disabled WITH_OPENEXR too.

Reason why EXR is great to include by default in Blender is its feature
to store unlimited layers and channels, and write this tile based. I
need the feature for saving memory; while rendering tiles, all full-size
buffers for all layers and passes are kept in memory now, which can go
into 100s of MB easily.
The code I commit now doesn't allocate these buffers while rendering, but
saves the tiles to disk. In the end is it read back. Overhead for large
renders (like 300 meg buffers) is 10-15 seconds, not bad.

Two more interesting aspects:
- Blender can save such multi-layer files in the temp directory, storing
it with .blend file name and scene name. That way, on each restart of Blender,
or on switching scenes, these buffers can be read. So you always see what was
rendered last. Also great for compositing work.
- This can also become an output image type for rendering. There's plenty of
cases where you want specific layers or passes saved to disk for later use.

Anyhoo, finishing it is another days of work, and I got more urgent stuff
now!

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